The mechanism used for generating the coercivity in permanent magnets currently under use may be enumerated by single magnetic domain particle type, nucleation type and pinning type mechanisms. Of these, the nucleation type coercivity generating mechanism has been introduced in order to account for generation of large coercivity in a sintered magnet having a crystal grain size not less than the single magnetic domain particle size, and is based on the theory that facility of nucleation of an demagnetizing field in the vicinity of the crystal grain boundary determines the coercivity of the crystal grain in question. This type of the magnet has peculiar magnetization properties that, while saturation of magnetization in the initial process of magnetization occurs at a lower impressed magnetic field, a magnetic field not less than the saturation magnetization needs to be applied to obtain sufficient coercivity. It may be presumed that the high magnetic field can drive off any demagnetizing field left in the crystal grain completely by a high magnetic field thus producing high coercivity. Examples of the magnet having the nucleation type coercivity generating mechanism include SmCo5-based or Nd—Fe—B-based sintered magnets.
The R-TM-B based permanent magnet has superior magnetic properties, and is finding a wide field of usages. There are a variety of manufacturing methods for the R-TM-B based permanent magnet, the most representative one being a sintering method and a rapid solidification method. The sintering method, as disclosed in Japanese Laying-Open Patent Kokai JP-A-59-46008, is a method consisting in pulverizing an ingot of a specified composition to fine powders of single crystals with a mean particle size of several μm, consolidating the powders to an optional shape under magnetic orientation in a magnetic field, and sintering the green compact to a bulk magnet. The rapid solidification method, disclosed in Japanese Patent Kokai JP-A-60-9852, is a method consisting in rapidly solidifying an alloy of a specified composition by a method such as roll quenching method to an amorphous state followed by heat treatment to precipitate fine crystal grains. The magnet alloy obtained by the rapid solidification method is usually powdered and are routinely mixed with a resin and molded to produce bonded magnets.
Rare earth magnetic powders having the coercivity generating mechanism of the pinning type, such as Sm2Co17, can be processed into magnetic powders suitable for bonded magnets simply by pulverizing a molten ingot of a pre-set composition. On the other hand, in rare earth magnetic powders having the coercivity generating mechanism of the nucleation type, practically useful coercivity is not produced unless the crystal grain size of the powdered particles is set so as not to be larger than the single magnetic domain particle size. Thus, as a manufacturing method in which the Nd2Fe14B crystal grain size in the powdered particles is less than the single magnetic domain particle size, there are currently used a rapid solidification method and a HDDR (hydrogenation-decomposition-dehydrogenation-recombination) method.